Optical switching, so-called photonic switching, is described in the publication "Technical Digest" 1990, International Topical Meeting on Photonic Switching, April 12-44, 1990, pages 176-178. The article describes an optical switching network which enables optical signals on the input side of the optical switch to be redistributed to different outputs on the output side of the switch. In order to ensure that two different signals on one and the same switch output will not obtain the same wavelength, since this would prevent the two signals from being distinguished from one another, a wavelength converter is arranged at the switch inputs. These wavelength converters are electrically operated converters. This is disadvantageous, since the optical transparency through the switch is lost. An optical transparent switch is capable of switching optical signals independently of the bit of rate and coding of the optical signals.
Described in the "Journal of Lightwave Technology", Vol. 8, No. 5, May 1990, pages 660-665, is an optical switch for wavelength multiplexed optical signals. Signals which arrive at the switch on one wavelength shall depart from the switch on another wavelength and optical modulators which comprise optical non-linear elements are used to this end. The non-linear effect is, however, very weak in practice and consequently the input light must be of very high intensity in order to achieve wavelength conversion. No optical non-linear devices which will operate effectively in practice are available at present, and consequently the authors of the article have replaced these devices with optical-electrical converters in the review of the experiments carried out. Such optical-electrical converters are disadvantageous for the reasons stated above.
In another issue of the publication "Journal of Lightwave Technology", Vol. 8, No. 3, March 1990, pages 416-421, there is described an optical switch with which wavelength conversion is effected with the aid of an intermediate frequency filter which operates outside the optical wavelength field. This arrangement thus has the limitations described above.
In certain applications an optical switch has to switch the input optical signals through the switch in a strictly blocking-free fashion. Strictly blocking-free fashion means that a signal at the input of the switch can find its way through the switch out into a free wavelength channel at the output link from the switch. By blocking is meant the simultaneous occurrence of two identical wavelengths on one and the same link.
Consider the following example which illustrates an optical switch wherein blocking occurs. Suppose the wavelength channels of an output link from the switch all are occupied except one. The free wavelength channel at the output corresponds to a wavelength .lambda..sub.1. Now suppose that there is a first signal having the wavelength .lambda..sub.1 at the input link and that this signal is switched to another output link than the one considered. Since the wavelength .lambda..sub.1 is occupied by the first signal a second signal, also having the wavelength cannot be used on the same input link. Accordingly the free output wavelength channel remains unused and the switch is said to be blocked.
To cancel the blocked condition above it is common to loosen the rather tight blocking-free requirement and instead operate the optical switch in a rearranged blocking-free fashion. In a rearranged blockig-free switch a wavelength channel which is already set up is shifted to a new wavelength selected amongst those which are possible to use for the already set up channels. Rearrangment may imply interruption in the transmission of already set up wavelength channels.
A disadvantage with known optical switches provided with opto-electrical converters is that the switches are dependent on the bit rate and code format of the optical signals to be switched. This impedes future extension and upgrading of existing networks, in which optical switches of such kind are used, since the switches must be changed if for example a higher bit rate or a new coding scheme of the signals should be introduced into the network.